Cereal is defined as any plant from the grass family that yields an edible grain (seed). Some of the more popular grains include barley, corn, millet, oats, quinoa, rice, rye, sorghum, triticale, wheat and wild rice. Further, cereal grains are considered a staple throughout the world because they are generally inexpensive, a readily available source of protein and have high carbohydrate content.
Comparing the various cereal grains, a high percentage of the world's food is rice based. The annual world rice production during a typical year is between 500 and 600 million metric tons. This amount of rice comes from over 50 countries throughout the world who contribute at least 100,000 tons of rice annually. The United States rice production has recently been over 8 million metric tons accounting for 1 and 2% of total world production. Although the consumption of rice in the U.S. is small by comparison to many Asian countries, consumption has increased 30% over the past 10 years. This is in part due to the general interest in rice for improving health.
On the other hand, wheat also makes up a high percentage of the world's food. In fact, wheat's status as a staple is second only to rice. One reason for the popularity of wheat is that, unlike other cereals, wheat contains a relatively high amount of gluten, the protein that provides the elasticity necessary for bread making. As such, wheat is typically ground into flour and used for various foods such as bread. Therefore, because wheat is generally ground up, it is easily fortified with iron and other minerals. Conversely, rice and other grains that are not typically ground into flour are more difficult to fortify.
One possible fortification method of cereals, such as rice, is to fortify the plant or grass from which the various cereal grains grow. The theory is that by adding minerals to the soil, the plant or grass will uptake those minerals and pass them on to the respective grains. A second possible fortification method of cereal grains is to coat actual kernels with desired vitamins and/or minerals.
Cereal grains have been fortified with many different vitamins and minerals. For example, vitamin A, vitamin C, vitamin E and B-complex vitamins have all been fortificants for cereal grains. Minerals used as fortificants have included iron, calcium, zinc, manganese, copper and other essential minerals. In considering various forms of iron and iron compounds, elemental iron, ferrous sulphate and ferrous fumarate have been used in the past as preferred iron fortificants. When selecting what iron fortificant to use, the color and taste of the iron compounds is a major consideration, especially when fortifying lightly colored foods. Therefore, a highly bioavailable form of iron may not be desirable to utilize because of resulting color changes and the unpalatability associated with a metallic taste. For example, though more soluble iron compounds such as ferrous sulfates are highly bioavailable, they often result in off-color and off-tasting grains.
The coating of rice and other grains for various purposes is not a new concept. In U.S. Pat. No. 5,702,745, a method of making a shelf-stable, ready-to-eat rice product is disclosed that involves coating pregelatinized rice grains with an emulsifier. The purpose of this coating is to provide a superior texture, appearance and flavor. In other words, the coating is not for delivery of vitamins and/or minerals. Further, in U.S. Pat. No. 4,687,669, a process for production of enriched rice or barley is disclosed. There, an oily or waxy coating is deposited on rice or barley. The coating is designed to remain intact while washing and to melt upon heating.
Additionally, fortifying grain products with iron is also not a new concept. In fact, in U.S. Pat. No. 4,931,292, the use of a certain type of complex iron(III)phosphate for iron fortification of grains is disclosed having good properties with respect to solubility near pH 1 (the pH found in the stomach), is bioavailable and almost colorless. The patent specifically mentions flours, breakfast cereals and rice as foods that may be fortified. An example or preferred embodiment of the invention is fortification by combining the food product with a complex of iron(III) phosphate having the formula Fe.sub.3 H.sub.8 (NH.sub.4) (PO.sub.4).sub.6.6H.sub.2 O. This compound is shown to be more bioavailable than carbonyl iron in humans. Further, the patent alleges that the appearance and taste of the fortified grain remains largely unchanged compared unfortified foods.
Fortification using chelates is an alternative way to increase bioavailability. The term "chelate" has often been misunderstood or applied in a general or catch-all fashion. A true chelate has a definite structure resulting from precise requirements of synthesis. Proper conditions must be present for chelation to take place, including proper mole ratios of ligands to metal ions, pH and solubility of reactants. For chelation to occur, all components must be in solution and have an appropriate electronic configuration in order for covalent bonding to develop.
Chelation can be confirmed and differentiated from mixtures of components by infrared spectra through comparison of the stretching of bonds or shifting of absorption peaks caused by bond formation. As applied in the field of mineral nutrition, there are two allegedly "chelated" products which are commercially utilized. The first is referred to as a "metal proteinate." The American Association of Feed Control officials (AAFCO) has defined a "metal proteinate" as the product resulting from the chelation of a soluble salt with amino acids and/or partially hydrolyzed protein. Such products are referred to as the specific metal proteinate, e.g., copper proteinate, zinc proteinate, etc. This definition does not contain any requirements to assure that chelation is actually present. On the basis of the chemical reactant possibilities, there are some real reservations as to the probability of chelation occurring to any great degree. For example, the inclusion of partially hydrolyzed proteins as suitable ligands and the term "and/or" in reference to such ligands implies that products made solely from partially hydrolyzed protein and soluble salts would have the same biochemical and physiological properties as products made from combining amino acids and soluble metal salts. Such an assertion is chemically incorrect. Partially hydrolyzed protein ligands may have molecular weights in the range of thousands of daltons and any bonding between such ligands and a metal ion may be nothing more than a complex or some form of ionic attraction, i.e., the metal drawn in close proximity to a carboxyl moiety of such a ligand.
While some products marketed as metal proteinates during the 1960's and 1970's were true chelates, this was prior to the adoption of the AAFCO metal proteinate definition. An analysis of products currently marketed as metal proteinates reveals that most, if not all, are mixtures of metal salts and hydrolyzed protein or complexes between metal salts and hydrolyzed protein. Most are impure products which are difficult to analyze and are not consistent in protein make-up and/or mineral content.
An amino acid chelate, when properly formed, is a stable product having one or more five-membered rings formed by reaction between the carboxyl oxygen, and the .alpha.-amino group of an .alpha.-amino acid with the metal ion. Such a five-membered ring is defined by the metal atom, the carboxyl oxygen, the carbonyl carbon, the .alpha.-carbon and the .alpha.-amino nitrogen. The actual structure will depend upon the ligand to metal mole ratio. The ligand to metal mole ratio is at least 1:1 and is preferably 2:1 but, in certain instances, may be 3:1 or even 4:1. Most typically, an amino acid chelate may be represented at a ligand to metal ratio of 2:1 according to the following formula: ##STR1##
In the above formula, when R is H, the amino acid is glycine which is the simplest of the .alpha.-amino acids. However, R could be representative of any other of the other twenty or so naturally occurring amino acids derived from proteins. These all have the same configuration for the positioning of the carboxyl oxygen and the .alpha.-amino nitrogen. In other words, the chelate ring is defined by the same atoms in each instance. The American Association of Feed Control Officials (AAFCO) has also issued a definition for an amino acid chelate. It is officially defined as the product resulting from the reaction of a metal ion from a soluble metal salt with amino acids with a mole ratio of one mole of metal to one to three (preferably two) moles of amino acids to form coordinate covalent bonds. The average weight of the hydrolyzed amino acids must be approximately 150 and the resulting molecular weight of the chelate must not exceed 800. The products are identified by the specific metal forming the chelate, e.g., iron amino acid chelate, copper amino acid chelate, etc.
The reason a metal atom can accept bonds over and above the oxidation state of the metal is due to the nature of chelation. In Formula I, it is noted that one bond is formed from the carboxyl oxygen. The other bond is formed by the .alpha.-amino nitrogen which contributes both of the electrons used in the bonding. These electrons fill available spaces in the d-orbitals. This type of bond is known as a dative bond or a coordinate covalent bond and is common in chelation. Thus, a metal ion with a normal valency of +2 can be bonded by four bonds when fully chelated. When chelated in the manner described the divalent metal ion, the chelate is completely satisfied by the bonding electrons and the charge on the metal atom (as well as on the overall molecule) is zero. This neutrality contributes to the bioavailability of metal amino acid chelates.
Amino acid chelates can also be formed using peptide ligands instead of single amino acids. These will usually be in the form of dipeptides, tripeptides and sometimes tetrapeptides because larger ligands have a molecular weight which is too great for direct assimilation of the chelate formed. Generally, peptide ligands will be derived by the hydrolysis of protein. However, peptides prepared by conventional synthetic techniques or genetic engineering can also be used. When a ligand is a di- or tripeptide a radical of the formula [C(O)CHRNH].sub.e H will replace one of the hydrogens attached to the nitrogen atom in Formula I. R, as defined in Formula I, can be H, or the residue of any other naturally occurring amino acid and e can be an integer of 1, 2 or 3. When e is 1 the ligand will be a dipeptide, when e is 2 the ligand will be a tripeptide and so forth.
The structure, chemistry and bioavailability of amino acid chelates is well documented in the literature, e.g. Ashmead et al., Chelated Mineral Nutrition, (1982), Chas. C. Thomas Publishers, Springfield, Ill.; Ashmead et al., Intestinal Absorption of Metal Ions, (1985), Chas. C. Thomas Publishers, Springfield, Ill.; Ashmead et al., Foliar Feeding of Plants with Amino Acid Chelates, (1986), Noyes Publications, Park Ridge, N.J.; U.S. Pat. Nos. 4,020,158; 4,167,564; 4,216,143; 4,216,144; 4,599,152; 4,774,089; 4,830,716; 4,863,898 and others. Further, flavored effervescent mixtures of vitamins and amino acid chelates for administration to humans in the form of a beverage are disclosed in U.S. Pat. No. 4,725,427.
One advantage of amino acid chelates in the field of mineral nutrition is attributed to the fact that these chelates are readily absorbed in the gut and mucosal cells by means of active transport as though they were solely amino acids. In other words, the minerals are absorbed along with the amino acids as a single unit utilizing the amino acids as carrier molecules. Therefore, the problems associated with the competition of ions for active sites and the suppression of specific nutritive mineral elements by others are avoided. This is especially true for compounds such as iron sulfates that must be delivered in relatively large quantities in order for the body to absorb an appropriate amount leading to possible nausea, diarrhea and other discomforts. Yet, because iron is such an important mineral to many physiological functions and because unfortified foods taken in by a typical person lack a sufficient amount of iron, fortification remains one of the best methods of affording people the minimum daily requirement of iron.
In view of the foregoing, it would be useful to provide a composition and method for fortification of a cereal grain kernel with iron, calcium, zinc and/or other minerals. More specifically, it would be useful to provide a composition and method for coating unpulverized or intact cereal grain kernels with amino acid chelates. Because of the increased bioavailability of metals when delivered as the closing member of an amino acid chelate, a smaller amount of metal may be used to fortify a cereal grain kernel, thereby reducing unwanted side effects and unpalatability.